1. Field of the Invention
The present invention relates generally to arrays of solar energy concentrators, in particular for space applications. It relates more particularly to an energy concentrator device employing solar cells.
2. Description of the Prior Art
The solar generator panels carried on a spacecraft such as a geosynchronous satellite generally comprise:
panels supporting an array of identical or different photovoltaic cells for converting solar energy into electrical energy, and
an array of identical or different solar cells forming strings of cells that are electrically connected and cover the surface of the support panel.
Solar generator panels of the above kind can be deployed in highly diverse configurations. Conventionally this means in a longitudinal succession parallel to a direction away from the body of the spacecraft and about which the generator turns to track the sun. However, to increase the electrical power available, it has been proposed to provide supplementary lateral panels in addition to the panels previously described. Also known in the art are configurations in which the panels are disposed in a transverse direction, i.e. in a direction transverse to the longitudinal direction previously cited along which extends the yoke that connects the generator to the body of the spacecraft and about which the generator turns to track the sun.
Energy concentrator arrays of the type mentioned hereinabove are described in the documents U.S. Pat. No. 6,188,012 and WO 00/79593 A1, for example.
However, anomalies corresponding to loss of sections of solar panels have been observed with this type of array.
Analysis of the space environment and tests on the ground have shown that the space environment can generate electrostatic charging and discharging phenomena in the panels. Further analyses and tests have shown that an electrostatic discharge between a solar cell of a panel and its cover glass can cause a short circuit between the cell and an adjacent cell, leading to a flow of solar generator output current between the cells. The energy conveyed by this current is then sufficient to destroy the insulative substrate of the cell support (which is generally made of Kapton™) and render it conductive.
To be more precise, the scenario leading to the anomalies observed proceeds in the following stages (see FIGS. 1a to 1e):
FIG. 1a shows two rows 1, 2 of cells of a solar generator panel 3. Each row includes a predetermined number of solar cells 4, 5 and the respective voltages at the terminals of each row are V1 and V2 volts.
Referring to FIG. 1b, because of the space environment, an inverted voltage gradient (IVG) appears between the solar cell connected to the electrical ground of the satellite (for example the cell 4) and its cover glass 6. Because of photo-emission, the cover glass is less negatively charged than the frame 7 (the rear face of the panel, and therefore in the shade), to which the cell is connected.
If the voltage between the solar cell 4 and the cover glass 6 exceeds a particular threshold, a primary electrostatic discharge occurs and generates a dense plasma 8, as shown in FIG. 1c. This discharge contains little energy and is in itself not a risk to the operation of the solar generator. The plasma created in this way propagates in the gap between adjacent cells.
However, if the voltage between these cells is higher than a threshold Vthreshold, the output current of the row can then flow between the cells via the plasma. This secondary arc can be maintained by the output current of the generator for a few seconds or more, as shown in FIG. 1d. 
It appears that the secondary arc then dissipates sufficient energy to heat the insulative substrate 9 to a temperature such that the high resistance polymer is converted into a material of very low resistance. This process (known as pyrolysis) makes the part 10 shown shaded in the figure completely conductive, which leads to a permanent short circuit between the two cells 4 and 5, and therefore between the two rows of cells 1 and 2, as shown in FIG. 1e, and also between the cell (4 or 5) and the electrical ground of the satellite, which causes the permanent destruction of a portion of the solar generator, so that it is incapable of delivering all of the output power to the equipment of the satellite.
One method of reducing the primary discharge risk consists of adding electrically conductive surfaces to the illuminated face of the solar generator and connecting them to the electrical ground of the satellite. Moreover, a method of reducing the risk of secondary discharge between adjacent rows would be to increase the gap between rows of photovoltaic cells. However, the space created in this way would reduce the ratio by which the panel is filled with cells and would then compromise the solar energy collecting power of the panel.
An object of the present invention is to remedy the above problems by proposing an energy concentrator device for spacecraft that employs solar cells and is of the type described above, and which reduces the risk of primary and secondary electrostatic discharge.